Injection molding nozzles which utilize a concentric pin or rod (a "valve pin") to shut off melt flow require an actuator to move the pin axially back and forth along a pin axis to open and close the melt nozzle. This can be problematic in single cavity molds where the melt inlet is generally opposite the outlet end of the nozzle. In such an arrangement, it is necessary to actuate the valve pin while diverting melt flow around the valve pin.
U.S. Pat. No. 4,729,733 (Schmidt) is illustrative of a prior art solution addressing the above problem. The Schmidt arrangement divides melt flow from a central inlet into two branches which extend around a centrally mounted hydraulic actuator and recombine below the actuator.
The Schmidt arrangement has numerous shortcomings. Firstly, the limited amount of space available restricts piston size requiring the use of a hydraulic actuator which is undesirable because of the possibility of hydraulic fluid leaking past the valve pin, thereby contaminating the melt.
Secondly, because of the proximity of the actuator to the branches (which act as a conduit for hot melt) and the inability to provide adequate cooling in the hot, restricted location, seal wear is problematic.
Additionally, the height of the Schmidt arrangement makes it undesirable for stack molds, particularly multi-level stack molds.
It is an object of the present invention to provide a valve pin actuator which is not restricted to a size which will fit within the centralized location required by the Schmidt arrangement.
It is a further object of the present invention to provide a valve pin actuator having a piston size large enough to allow the use of pneumatic actuation.
It is a still further object of the present invention to provide a valve pin actuator having a piston, the location of which is not surrounded by hot melt, thereby avoiding heat damage to any seals.
It is also an object of the present invention to provide a valve pin actuator having a relatively low height to enable its use in stack mold arrangements.